fix: always return 0 from source tree validator in default path

Missing source trees (tar present but not extracted) are non-fatal — the
cookbook fetches/extracts them during build. Also fixed the
--missing-paths-only branch (previous commit).

local/scripts/validate-source-trees.py

@@ -134,8 +134,7 @@ def main():
     if missing_entries:
         release = args.release or "<release>"
         print(f"\nTo restore:  ./local/scripts/restore-sources.sh --release={release}")
-        return 1
-    print("All source trees present.")
+    print("Source tree validation complete.")
     return 0